1. Field of the Invention
The invention relates to an overvoltage protection element with a housing, with at least one overvoltage limiting component which is located in the housing, with terminal elements for electrical connection of the overvoltage protection element to the current or signal path which is to be protected, and with a state display which has a display element for displaying the state of the overvoltage protection element.
2. Description of Related Art
The overvoltage limiting components which are located in the overvoltage protection element, especially varistors and spark gaps, are subject to progressive degradation, i.e., an aging-induced change of the parameters of the overvoltage limiting components. The degradation process can lead over time to failure of the surge arrester during a diversion process or also under network conditions.
Varistors “age” both at the prevailing operating voltage and also for any diversion process which occurs. With progressive degradation first of all the resistance in the “nonconductive state” of the varistors decreases so that the leakage current which occurs increases; this leads to heating of the varistor. This heating of the varistor is used in the known overvoltage protection elements to trigger an isolation mechanism starting from a given temperature, which electrically isolates the varistor from the current or signal path to be protected.
In contrast to varistors, spark gaps “age” generally only during the diversion process. The arc which prevails between the electrodes of a spark gap in the case of diversion leads, on the one hand, to damage of the electrodes themselves, on the other hand, also to damage of the insulation parts surrounding the electrodes. Here, the state of the inner opponents of a spark gap is generally unknown and can also be determined only to a very limited degree by measurements within certain test cycles. In spark gaps, there is generally no isolation mechanism, as in varistors.
German Patent Application DE 44 13 057 A1 discloses a method for detection of lightning stroke or surge currents in which a premagnetized data medium is located in the three-dimensional vicinity of a conductor through which the lightning stroke or surge current has flowed; this leads to a change of the magnetization of the data medium. The change of the magnetization of the data medium can be detected and evaluated by means of a reader into which the data medium must be plugged. On the one hand, this method requires active implementation by correspondingly trained personnel, on the other only the magnetization or magnetization change which has occurred at maximum can be measured and evaluated so that for example several smaller pulses before or after the maximum surge current are not recognized. With this method thus only the maximum current intensity of a lightning stroke or surge current can be determined; a conclusion about the state of an arrester however cannot be easily drawn.
German Utility Model DE 20 2004 006 227 U1 and corresponding U.S. Pat. No. 7,411,769 B2 disclose an overvoltage protection element in which the state of a varistor is monitored according to the principle of a temperature switch so that when the varistor overheats a solder connection provided between the varistor and the interrupting element is broken; this leads to electrical isolation of the varistor. Moreover, when the solder connection is broken, a plastic element is pushed by the reset force of a spring out of a first position into a second position in which the interrupting element which is made as an elastic metal tongue is separated thermally and electrically from the varistor by the plastic element so that an arc which may prevail between the metal tongue and the contact site of the varistor is extinguished. Since the plastic element has two colored markings located next to one another, it acts additionally as an optical state display, as a result of which the state of the overvoltage protection element—serviceable (green) or isolated (red)—can be read directly on site.
German Patent Application DE 10 2007 006 617 B3 discloses an overvoltage protection element with two overvoltage limiting components which are located in one housing, in which the two overvoltage limiting components, which can be two varistor disks, are isolated individually when overheated. The breaking of one or both solder connections is indicated by an optical state display made as a rotary display. Thus, the three different states can be displayed—both varistors serviceable, one varistor no longer serviceable and isolated, neither varistor serviceable any longer and isolated. In this overvoltage protection element, the optical state display, however, only displays whether one or both varistors are serviceable or not. There is no additional display of the state of one still serviceable varistor in the known overvoltage protection elements.